Electron beam apparatuses, in particular a scanning electron microscope (also referred to as SEM below) and/or a transmission electron microscope (also referred to as TEM below), are used to examine objects (also referred to as samples) in order to obtain knowledge in respect of the properties and behavior of the objects under certain conditions.
In an SEM, an electron beam (also referred to as primary electron beam below) is generated by means of a beam generator and focused on an object to be examined by way of the beam guiding system. An objective lens is used for focusing purposes. The primary electron beam is guided in a grid-shaped manner over the surface of the object to be examined by way of a deflection device. Here, the electrons of the primary electron beam interact with the object to be examined. Interaction particles and/or interaction radiation is/are generated as a result of the interaction. By way of example, the interaction particles are electrons. In particular, electrons are emitted by the object—the so-called secondary electrons—and electrons of the primary electron beam are scattered back—the so-called backscattered electrons. The secondary electrons and backscattered electrons are detected by means of at least one particle detector. The particle detector generates detection signals, which are used to generate an image of the object. Thus, an image of the object to be examined is obtained. By way of example, the interaction radiation comprises x-ray radiation and/or cathodoluminescence radiation. The interaction radiation is detected by means of at least one radiation detector, which generates detection signals. By way of example, these detection signals are used to generate spectra, by means of which properties of the object to be examined are determined.
In a TEM, a primary electron beam is likewise generated by means of a beam generator and focused on an object to be examined by means of a beam guiding system. The primary electron beam passes through the object to be examined. When the primary electron beam passes through the object to be examined, the electrons of the primary electron beam interact with the material of the object to be examined. The electrons passing through the object to be examined are imaged onto a luminescent screen or onto a detector—for example in the form of a camera—by a system having an objective. By way of example, the aforementioned system additionally also comprises a projection lens. Here, imaging can also take place in the scanning mode of a TEM. Such a TEM is generally referred to as STEM. Additionally, provision can be made for detecting electrons scattered back at the object to be examined and/or secondary electrons emitted by the object to be examined by means of a further detector in order to image an object to be examined.
The integration of the function of an STEM and an SEM in a single particle beam apparatus is known. It is therefore possible to carry out examinations of objects with an SEM function and/or with an STEM function using this particle beam apparatus.
Furthermore, the prior art has disclosed the practice of analyzing and/or processing an object in a particle beam apparatus using, firstly, electrons and, secondly, ions. By way of example, an electron beam column having the function of an SEM is arranged at the particle beam apparatus. Additionally, an ion beam column is arranged at the particle beam apparatus. Ions used for processing an object are generated by means of an ion beam generator arranged in the ion beam column. By way of example, material of the object is ablated or material is applied onto the object during the processing. The ions are additionally or alternatively used for imaging. The electron beam column with the SEM function serves, in particular, for further examination of the processed or unprocessed object, but also for processing the object.
In the above-described particle beam apparatuses, provision is made of accelerating the charged particles of the particle beam to a certain energy. More precisely, provision is made of accelerating the electrons of the primary electron beam and/or the ions of the ion beam to a certain energy. This is explained below on the basis of electrons of a primary electron beam. An analogous statement applies to ions of an ion beam.
Electrons are generated by means of a beam generator both in an SEM and in a TEM. The electrons emerge from the beam generator and form the primary electron beam. The electrons are accelerated to a potential due to a potential difference between the beam generator and an anode. To this end, the beam generator is usually supplied with negative high voltage. By way of example, in the case of an SEM, this lies in the region of 0 V to (−50) kV in relation to the ground potential. By way of example, in the case of a TEM, the high voltage lies in the region of (−5) kV to (−4) MV in relation to the ground potential. In order to obtain a desired maximum resolution or a desired contrast and/or in order to adjust to a desired object thickness passable by radiation and/or a limit to the damage of the object, the practice of setting the high voltage to a certain value is known. In order, furthermore, to obtain a good resolution in the final images provided by the particle beam apparatuses, it is desirable to keep the voltage applied to the beam generator as stable as possible. Expressed differently, it is desirable for the high voltage provided by a high-voltage supply unit not to be subject to variations which would restrict a good resolution in the final images.
The prior art has disclosed a high-voltage supply unit for a particle beam apparatus, which has an AC voltage source which is adjusted by an amplitude regulator by way of a desired setpoint value of the high voltage and the output voltage of which is supplied to a step-up transformer. The step-up transformer steps up the AC voltage. The output voltage of the step-up transformer is fed to a Cockcroft-Walton generator in turn, the latter multiplying the output voltage of the step-up transformer. The high voltage arising thus is smoothed by way of a filter member or a plurality of filter members made of resistors and capacitors. The high voltage smoothed thus is fed to the amplitude regulator via a measurement resistor. Variations in the smoothed high voltage can be registered by way of a capacitive divider consisting of a first capacitor and a second capacitor, and said variations can be fed to an amplifier. The amplifier can provide an output signal which is fed to the amplitude regulator and which acts in anti-phase in relation to the variations of the smoothed high voltage. The variations of the smoothed high voltage can be additionally damped in this manner.
The high-voltage supply units used in the particle beam apparatuses are disadvantageous in that they only enable a 1-quadrant operation. This means that the known high-voltage supply units can only provide voltage values which are assignable to a single quadrant. Then, this is also referred to as a unipolar voltage supply unit, which only enables a load current in one direction. This is explained in greater detail with reference to FIG. 1. FIG. 1 shows a schematic illustration of a current-voltage behavior, in which the current is plotted on the abscissa axis and the voltage is plotted on the ordinate axis. The abscissa axis and the ordinate axis intersect at an origin. The origin separates negative current values (I−) from positive current values (I+). Moreover, the origin separates negative voltage values (U−) from positive voltage values (U+). The abscissa axis and the ordinate axis separate 4 quadrants from one another, namely a first quadrant I, a second quadrant II, a third quadrant III, and a fourth quadrant IV. In the case of the unipolar voltage supply unit, only voltage values which are assignable to a single one of the aforementioned quadrants are provided. However, a 4-quadrant operation is desirable such that it is possible to provide voltage values in all four quadrants (cf. FIG. 1). The rectangle depicted in FIG. 1 is delimited by a maximumly obtainable positive voltage U+max, a maximumly obtainable negative voltage U−max, a maximumly obtainable positive current I+max, and a maximumly obtainable negative current I−max. The work range of a desired bipolar voltage supply unit should lie within the depicted rectangle. Accordingly, a bipolar voltage supply unit enabling a load current in two directions is desired. In the case of further application forms, it is desirable to provide at least a 2-quadrant operation, by means of which a unipolar voltage supply unit can be formed.
Accordingly, it is desirable to provide a high-voltage supply unit and a circuit arrangement for generating a high voltage for a particle beam apparatus and a particle beam apparatus with a high-voltage supply unit, which enable a 4-quadrant operation or a 2-quadrant operation.